1. Field of the Invention
The present invention relates to using of a magnetic field, and more particularly, to a method and apparatus for using a magnetic field to check whether disturbance of a magnetic field exists and to use the check result.
2. Description of the Related Art
Hereinafter, a conventional method of calibrating a magnetic compass (hereinafter referred to as a compass), which is mounted in a mobile robot so as to recognize an azimuth of the mobile robot, will be described with reference to the attached drawings.
In general, the compass mounted in the mobile robot must be calibrated so as to reduce the distortion of the azimuth thereof caused by a magnetic field generated from the mobile robot.
FIG. 1 is a view for showing a circle formed on x- and y-axes using magnetic field values sensed by magnetic field sensors in a linear magnetic flux environment.
The compass in the mobile robot, for example, includes two magnetic field sensors (not shown) which are disposed to be orthogonal to each other. When the magnetic field values sensed by the magnetic field sensors are projected on a 2-dimensional plane with the rotation of more than 360° of the mobile robot, a circle as shown in FIG. 1 is formed. Here, the azimuth of the compass corresponds to an angle at which coordinates on the circle form with x- or y-axis of FIG. 1.
FIG. 2 is a view for showing a circle formed on x- and y-axes using binary magnetic field values sensed by magnetic field sensors in a linear magnetic flux environment.
The magnetic field values sensed by the magnetic field sensors and then a digital computer substantially exist in the binary digit form. Thus, when sensed positive magnetic field values are projected on a 2-dimensional plane, the center of the circle may be offset as shown in FIG. 2. Here, the offset of the center of the circle may also be caused due to disturbance of the magnetic field.
FIG. 3 is a view for showing a circle formed on x- and y-axes using binary magnetic field values sensed by magnetic field sensors when disturbance of a magnetic field exists.
When the binary magnetic field values sensed within a curved magnetic flux in which the disturbance of the magnetic field exists are projected on a 2-dimensional plane, the center of the circle has an offset and the circle is distorted, as shown in FIG. 3.
Here, the calibration of the compass means the work of transforming a circle with the offset at its center or a distortion as shown in FIG. 2 or 3 into the circle of FIG. 1. For this purpose, the conventional calibration method uses a one-to-one function or calibration parameters. However, since the surroundings of a mobile robot vary with the traveling of the mobile robot, the one-to-one function or the calibration parameters cannot be continuously used.
FIG. 4 is a view for a 2-dimesionally showing an exemplary traveling figure of a mobile robot. Here, dotted lines denote a magnetic flux.
In general, in the conventional calibration method, it is supposed that a linear magnetic flux predominately exists in a current location of a compass to be calibrated. For example, as shown in FIG. 4, a compass mounted in a mobile robot 4 is calibrated in a predetermined location 2 in which a linear magnetic flux predominates, using the conventional calibration method. Next, the compass moves in a direction indicated by an arrow. Here, the linear magnetic flux appears in the Earth's magnetic field. However, the compass to be calibrated using the conventional calibration method may be located in an indoor environment in which a human resides or in an outdoor environment around which many kinds of metal such as steel concrete or the like exist. In this case, a magnetic field is distorted by a metal or a magnetic substance. Thus, the above supposition of the conventional calibration method cannot be given.
FIG. 5 is a view for a 2-dimensionally showing an exemplary traveling figure of a mobile robot. Here, a linear magnetic flux may be changed into a curved magnetic flux due to disturbance of the magnetic field in an indoor environment or in an outdoor environment around which many kinds of metal exist.
As shown in FIG. 5, when a compass mounted in a mobile robot 8 is calibrated in an arbitrary location 6 within a curved magnetic flux using the conventional calibration method, the compass may not be accurately calibrated. As a result, the calibrated compass may not accurately reflect a traveling direction of the mobile robot 8.
U.S. Pat. No. 6,014,610 discloses a method of preventing compass data used for calibration from being distorted. In the disclosed method, when disturbance of a magnetic field is doubtful, a compass transiently stops operating. Next, when the magnetic signature of a mobile body changes, the compass is recalibrated. The disclosed method does not present a counterplan for inaccurate calibration occurring in an environment affected by a magnetic disturbance.
U.S. Pat. No. 6,301,794 discloses a method of calibrating in real-time a magnetic field varying when a mobile body with a compass travels. In the disclosed method, the circle of FIG. 3 may be changed into the circle of FIG. 1. However, the azimuth of the compass cannot be calibrated. In other words, a point on the distorted circle of FIG. 3 may correspond to a point on the circle of FIG. 1 on a one-to-one basis. However, whether the corresponding points can form a right azimuth is not sure. Thus, the disclosed method cannot obtain a more accurate azimuth than a calibration method performed within a magnetic field in which a linear magnetic flux flows.
In addition, a high-priced magnetic field measurer is required to check whether a magnetic disturbance exists. Moreover, in a case where the high-priced magnetic field measurer is mounted in a mobile robot, the high-priced magnetic field measurer hinders the practical use of the mobile robot.